Author/Authors :
A. ANSARI Civil Eng. Department - University of Tehran , Asa. NOORZAD Civil Eng. Department - University of Tehran , Ali NOORZAD Chairman - Iranian Committee on Large Dams
كليدواژه :
Seismicity , Hazard analysis , Uncertainty , Fuzzy mathematics
چكيده لاتين :
Our knowledge about earthquake, which is considered as a natural hazard, is highly conjugated with
uncertainty. This uncertainty could be categorized into two classes. First, earthquake is a random
event. It means that predicting the characteristics of the future seismic motion based on the current
information is a matter of randomness, because there is not a comprehensive data from all influencing
factors involved in generating seismic motion. From another point of view, not only there is very
limited information, but also the available data are not precise. It means that our knowledge about
earthquake is vague. This vagueness is not like randomness.
For years, probability theory was the only mathematical framework for considering all kind of
uncertainties. But by progressing of engineering knowledge and increasing the complexity of
problems, it is necessary to study the influence of information fuzziness in the response of dynamic
systems.
Concerning with the problem of assessment of seismic hazard in dams, there are different
sources of uncertainty. The data, upon which the seismicity of a region (i.e. earthquake catalogs and
geological information) is determined, have different level of vagueness. For example, the accuracy of
magnitude is not identical for all span of the catalog and on the other hand, the geological information
is often qualitative and quantifying them brings a great deal of uncertainty.
In the present paper, uncertainty of different parameters in magnitude probability density
function is modeled by the help of fuzzy mathematics. In this regard, the effects of uncertainty of
maximum and minimum magnitude in the catalog in addition to the effect of vagueness of earthquake
magnitudes are studied separately. It is concluded that the influence of mmax uncertainty is negligible
while this is not the case for m0 which is minimum or threshold magnitude and for magnitude of each
event. There is also some kind of uncertainty for the seismicity parameter β which is due to the
selection of estimation method. This uncertainty is crucial important and it is shown that it has a
drastic influence on the magnitude distribution function. It is also demonstrated that the effect of
different parameters’ uncertainty is considerable in the low and moderate magnitude range of the
catalog.
